This invention relates to an inspection apparatus using magnetic resonance (hereinafter referred to as the "MRI apparatus"). More particularly, it relates to the construction of a gradient coil for reducing an eddy current that occurs with the change of a gradient magnetic field with time, and to an RF probe.
X-ray CT and ultrasonic diagnostic apparatuses have been widely utilized in the past as apparatuses for non-destructively inspecting the internal structure of the human body. Further, it has become possible in recent years to conduct similar inspections by using a magnetic resonance phenomenon and to acquire those data which cannot be obtained by X-ray CT and the ultrasonic diagnostic apparatuses.
MRI is an imaging diagnostic method utilizing the phenomenon that an atomic nucleus of a hydrogen atom or the like in an inspected object placed inside a static magnetic field resonates with a radio frequency (RF) magnetic field having a specific frequency proportional to the intensity of the static magnetic field. This is the method which can obtain an arbitrary tomogram of the human body and can also provide data on a blood flow and biological functions such as metabolism. Further, brain function imaging utilizing the increase of the blood flow quantity and the difference of magnetical properties of hemoglobins in the blood has been started recently, and studies on the brain functions have made a new turn.
In the apparatuses utilizing the magnetic resonance, signals from the inspected object must be separated and discriminated. One of the known methods for this purpose comprises applying a gradient magnetic field to the inspected object, making the static magnetic field applied to each part of the inspected object different from that of others and acquiring position data, for example. The fundamental principle of the apparatuses of this kind is described in "Journal of Magnetic Resonance", Vol. 18, 1975, pp. 69.
Recently, attempts have been made to measure the dynamic motion of the heart and activated regions of the brain by a ultra-high speed imaging method such as an echo-planar method by using such an apparatus. In the ultra-high speed imaging method, a method which uses an exclusive small gradient coil for generating a gradient magnetic field in the proximity of the inspected object has been proposed on the basis of the fact that it is primarily the head or the heart which is to be inspected (JP-A-1-227747).
In conjunction with the eddy current, too, it is known that the generation quantity of the eddy current can be restricted by reducing the size of the gradient coil and thus increasing the distance between this coil and a conducting portion of a magnet for generating the static magnetic field as static magnetic field generation means.
However, it is difficult to completely eliminate the eddy current by such a technology alone, and the eddy current must be further reduced. One of the effective methods is the one that uses a self-shielded coil. This method disposes a correction coil for generating an additional magnetic field having an opposite polarity in addition to a main coil for generating a main gradient magnetic field so that the magnetic field generated by the gradient coil can be reduced to a substantially negligible level at the conducting portion of the magnet. For, if the magnetic field is not generated at the conducting portion, the eddy current is not generated, either.
FIGS. 3 and 4 of the accompanying drawings are sectional views each showing the arrangement of the coil according to the prior art.
Next, an example of the prior art apparatuses will be explained with reference to FIG. 3. According to the prior art, a small gradient coil 220 is disposed inside a bore of a magnet 211 for generating the static magnetic field. A strong gradient magnetic field can be generated more easily by reducing the size of the gradient coil, but it is difficult to completely eliminate the eddy current. Therefore, the prior art technology adds a gradient coil 223 for correction outside a main gradient coil 221 so as to offset the eddy current and uses it as a unitary gradient coil. These gradient coils 221 and 223 are movable on a supporter 214 with a bed 213. However, because these coils are disposed close to each other, current-to-magnetic field transformation efficiency is low. Moreover, the bed 213 and the supporter 214 must be ruggedly produced so as to support the weight of the coils.
When a cross-section is imaged at a high speed of about dozens of mili-seconds to several seconds by the brain function imaging methods which have been actively made in recent years, a strong gradient magnetic field having quick rise and fall must be applied at a high speed to the inspected object. In other words, a large current having quick rise and fall must be caused to flow through the gradient coil and a power supply unavoidably becomes great in size.
Generally, the gradient coil imparts gradients to the homogeneous static magnetic field generated by the magnet, in two directions, that is, the direction of the static magnetic field and a direction orthogonally crossing the former, by using a plurality of coils. The coil for imparting the gradient in the direction of the static magnetic field is the one which is disposed in such a fashion that the direction of the magnetic field generated by it at the coil center lies in the direction of the static magnetic field (refer to "NMR Medicine", The Japanese Society of Magnetic Resonance in Medicine (Editor), Chapter 4, pp. 73-82, Maruzen (1991)). The direction of the magnetic field generated by the coil for imparting the gradient in the direction orthogonally crossing the direction of the static magnetic field, at the coil center, is different depending on the type of the magnet used.
In the MRI apparatus using a magnet (permanent magnet) for generating a homogeneous static magnetic field in the direction orthogonally crossing the axial direction of a cylinder of a cylindrical inside space for loading the human body, the coil for imparting a gradient of a magnetic field in the direction orthogonally crossing the direction of the static magnetic field is the one which is disposed in such a fashion that the direction of the magnetic field generated by it at the coil center lies in the direction of the static magnetic field, in the same way as the coil for imparting the gradient to the magnetic field in the direction of the static magnetic field (JP-A-64-64638).
On the other hand, in the MRI apparatus using a magnet (super-conducting magnet) for generating a homogeneous static magnetic field in the direction of the cylinder axis of the cylindrical inside space for loading the human body, the coil for imparting the gradient to the magnetic field in the direction orthogonally crossing the direction of the static magnetic field is the one which is disposed in such a fashion that the direction of the magnetic field generated by it at the coil center lies in a direction orthogonal to the direction of the static magnetic field, as typified by a saddle coil ("NMR Medicine", The Japanese Society of Magnetic Resonance in Medicine (Editor), Chapter 4, pp. 73-82, Maruzen (1991)). The gradient magnetic field generated by the gradient coil imparts three-dimensional position data to the MR image, and the spatial gradient of the magnetic field intensity must be linear in the imaging region.
Generally, the smaller the size of the coil, the smaller becomes power for driving the coil but at the same time, the linear region of the gradient magnetic field becomes smaller. For this reason, there is the limit to the reduction of power for driving the coil.
In contrast, a method is known which disposes a small local gradient coil, which is smaller than the main gradient coil, in the proximity of the inspected object in addition to the main gradient coil, and can execute high speed imaging without requiring a large scale power supply (JP-A-2-80031). This method is based on the principle that because the intensity at a certain point of the magnetic field generated by the current is inversely proportional to the square of the distance between the current and the certain point due to the Biot-Savart's law, smaller power is needed by disposing the coil closer to the object for imaging in order to generate a magnetic field having the same intensity.
When brain functional imaging is carried out using the local gradient coil according to the prior art in the MRI apparatus using a super-conducting magnet for generating the homogeneous static magnetic field in the direction of the cylinder axis of the cylindrical inside space for loading the human body, the positional relationship between the human body and the local gradient coil is such as shown in FIGS. 11A and 11B. FIG. 11A is a plan view when the local gradient coil is viewed from the front surface of the human body, and FIG. 11B is a front and perspective view when the local gradient coil is viewed from the human head. In the drawings, the direction of the static magnetic field lies in the z direction, and the direction of the body's axis is brought into conformity with the direction of the static magnetic field. The human body is loaded into a bobbin 16 for supporting the local gradient coil. An RF probe 14 for transmitting and receiving a radio frequency magnetic field to and from the human body may be of a type which executes both transmission and reception or of a type which executes transmission and reception by separate probes.
As shown in FIG. 12, coils 11 and 12 which impart the gradient to the magnetic field in the x and y directions as the directions orthogonally crossing the direction of the static magnetic field are disposed in such a manner that the direction of the magnetic fields generated by these coils at their centers lies in the direction orthogonal to the direction of the static magnetic field. In FIG. 11A, among the conducting wire constituting the coil 12 for imparting the gradient to the static magnetic field in the y direction, it is the conducting wire 12-1 positioned near the imaging area that generates a magnetic field effective for imparting the gradient to the static magnetic field, which lies in the z direction, in the y direction.
FIG. 13 is a perspective view showing only four coils 12 among a plurality of coils constituting the local gradient coil, which impart the gradient to the static magnetic field in the y direction. When a current 31 flows through the conducting wire 12-1 near the imaging area in the direction indicated by an arrow, a magnetic field 33 develops in a right hand screw direction in the flowing direction of the current around the conducting wire 12-1. When the magnetic fields 33 generated by the current 31 flowing through the four coils are combined in the imaging area, they become the sum 34 of the magnetic fields, and the gradient in the y direction is imparted to the static magnetic field in the z direction. Since the current flows through the conducting wire 12-2 spaced apart from the imaging area in the direction opposite to the conducting wire 12-1, a magnetic field 35 develops in such a manner as to offset the magnetic field generated by the conducting wire 12-1. This also holds true of the coil 11 for imparting the gradient in the x direction to the static magnetic field. It is the conducting wire 11-1 positioned near the imaging area that generates the magnetic field effective for imparting the gradient in the x direction to the static magnetic field in the z direction.
When the magnetic field crossing the conductor surface changes time-wise, an eddy current develops on the conductor surface in such a manner as to generate a magnetic field which inhibits such a change, according to the Faraday's law on electromagnetic induction. In the MRI apparatus, the magnet for generating the static magnetic field is made of the conductor. Generally, a cylindrical conductor shield is disposed inside the magnet. Therefore, the eddy current occurs on the conductor surface which is the nearest to the cylindrical inside space for loading the human body among the conducting portion of the magnet or on the cylindrical conductor shield at the rise and the fall of the gradient magnetic field.
FIG. 14A is a perspective view showing the cylindrical conductor shield 41, the y direction gradient coil 12 and a varying magnetic field 42 occurring at the rise of a gradient magnetic field according to the prior art example, and FIG. 14B is a perspective view showing the eddy current which is generated on the cylindrical conductor shield 41 by the varying magnetic field 42 occurring at the rise of the gradient magnetic field according to the prior art example. The direction of the varying magnetic field 42 generated by the coil 12 for imparting the gradient to the magnetic field in the y direction, at the rise of the gradient magnetic field lies in a direction orthogonally crossing the sheet surface of the cylindrical conductor shield 41. The direction of the magnetic field generated by the coil for imparting the gradient to the magnetic field in the x direction is the same, too. Accordingly, when the gradient magnetic field is switched at a high speed during high speed imaging, the eddy current 43 is generated on the cylindrical conductor shield 41 by the varying magnetic fields generated at the rise and the fall of the gradient magnetic field.